1. Field of the Invention
The present invention relates to a non-contact tag antenna for transmission and reception to/from an RFID reader/writer and an RFID system using the same.
2. Description of the Related Art
An RFID (Radio Frequency Identification) system is a system which can read information in a tag with a reader/writer by transmitting a signal of about 1 W from the reader/writer with the use of a radio signal in UHF band (860 to 960 MHz); receiving the signal with a tag antenna on the tag side; processing the received signal with a chip; and sending back a response signal to the reader/writer side.
Various propositions have been made as such a system. Among the propositions, a configuration using a tag antenna is known (e.g., Japanese Patent Application Laid-Open Publication Nos. 2004-334432 and 2005-216077).
Although dependent on the gain of the tag antenna, an operation voltage of a chip and a surrounding environment, the communication distance thereof is on the order of 3 m.
FIG. 1 is a diagram showing a configuration example of a tag, with the tag having a dipole antenna 1 as an example of an antenna and a chip 2 connected a feeding point. In other words, the tag includes the antenna 1 with a thickness on the order of 0.1 mm and the LSI chip 2 (about 1 mm square, on the order of 0.2 mm thick) connected to the antenna feeding point.
FIG. 2 shows an equivalent circuit of the tag. The antenna 1 can be equivalently shown by parallel connection of a resistance Ra (e.g., 500 Ω) and an inductance La (e.g., 40 nH) while the LSI chip 2 can be equivalently shown by parallel connection of a resistance Rc (e.g., 1200 Ω) and a capacitance Cc (e.g., 0.7 pF).
When represented on an admittance chart shown in FIG. 3, the antenna 1 and the LSI chip 2 are located in positions represented by A and B shown in FIG. 3, respectively. Therefore, by connecting the antenna 1 and the LSI chip 2 in parallel, the capacitance and the inductance are resonated and matched at a desired resonance frequency F0 as understood from equation 1, and the received power of the antenna 1 is sufficiently supplied to the chip 2 side.
                              f          ⁢                                          ⁢          0                =                  1                      2            ⁢            π            ⁢                          LC                                                          (        1        )            
As a basic antenna used as the tag antenna, although a dipole antenna with a length of about 145 mm as shown in FIG. 1 is conceivable, as shown in FIG. 3, an impedance is f=953 MHz, a real part impedance Ra=72 Ω and an imaginary part=0. However, since the RFID tag antenna requires very high Ra which is on the order of 500 to 2000 Ω, Ra must be increased.
Therefore, as shown in FIG. 4, an antenna (folded dipole antenna) is used which has a folded dipole portion 3 with a length of about 145 mm, and it is well known that Ra can be increased to about 300 Ω to 500 Ω, although varied by a line width.
In the admittance chart of FIG. 3, an example of Ra=500 Ω is shown by C in the figure. As shown in FIG. 5, by connecting inductance 4 in parallel with the folded dipole antenna, anticlockwise rotation is made on the admittance chart of FIG. 3 to give an imaginary number component (Ba=−1/ωLa) of an absolute value which is the same as the chip (Bc=ωCc).
The longer the inductance length is, the smaller the La value is and the greater the rotation amount is. Therefore, the imaginary number component Bc of the chip and the imaginary number component Ba of the antenna have the same magnitude and are cancelled and resonated. This cancellation of the imaginary number components is the most important factor for the RFIC tag antenna design. On the other hand, although the resistance Rc of the chip and the antenna radiation resistance Ra are desired to be matched, the resistances are not needed to be matched precisely.
The tag is typically linearly polarized and the antenna on the reader/writer side is circularly polarized (rightward polarization or leftward polarization, an axial ratio=about 0.5 to 3 dBi) in order to be able to receive regardless of the direction to which the tag faces.
As described above, in a typical RFID system, since an antenna on the reader/writer side is circularly polarized and the tag side is linearly polarized, a communication distance is the same regardless of the sides of the tag. If the tag is configured with complete circular polarization, although a communication distance is increased in a direction identical to a rotation direction of circular polarization on the reader/writer side, when the tag is reversed, a communication distance is reduced drastically since a rotation direction is reversed.
In the RFID system, the tag is attached to some sort of object to perform reading or writing, and the target object for attaching the tag has a certain degree of size, of course. When using a conventional linearly polarized tag as the tag, if reading is attempted from the back side which is opposite to the attaching side of the tag, a communication distance is decreased by the size of the attached object.
For example, when a conventional linearly polarized tag has a communication distance of 3 m, if a target object for attaching the tag has a thickness of about 1.4 m, although the communication distance from the tag-attached side is still 3 m, the communication distance from the back side of the target object for attaching the tag is 3 m−1.4 m=1.6 m, which is practically problematic.
If the tag has appropriate linearly polarized and appropriate circularly polarized components, for example, when the tag is faced to the reader/writer, the communication distances can be differentiated between the sides of the tag such that the communication distance is 3.9 m on the front side (direction identical to the direction of circular polarization) and 2.1 m on the back side (direction not identical to the direction of circular polarization). In the above example, if the front side with the increased distance is attached to the attached object, since the communication distance from the side with the tag attached is 2.1 m and the communication distance from the opposite side of the attached object is 3.9 m 1.4 m=2.5 m, the communication distances from the attached object can be made about the same distances in the practical use.
However, an antenna with such a characteristic does not exist. In consideration of the practical operation, the antenna must be to the extent of a card size. In the LSI chip 2, the resistance Rc (e.g., 1200 Ω) and the capacitance Cc (e.g., 0.7 pF) must be matched.